1. Field of the Invention:
The present invention relates to a vehicle hydraulic pressure control apparatus that executes an anti-skid control (hereinafter referred to as “ABS control”) for preventing an excessive slip of a wheel.
2. Description of the Related Art:
Heretofore, a brake hydraulic pressure control apparatus that controls a brake hydraulic pressure in a wheel cylinder (hereinafter referred to as “wheel cylinder hydraulic pressure”) to execute the ABS control has widely been mounted on vehicles. In general, the brake hydraulic pressure control apparatus is provided with a normally-open solenoid valve (pressure-increasing valve) disposed in a hydraulic circuit between a master cylinder generating a brake hydraulic pressure (hereinafter referred to as “master cylinder hydraulic pressure”) according to a brake operation by a driver and the wheel cylinder and a normally-closed solenoid valve (pressure-reducing valve) disposed in a hydraulic circuit between the wheel cylinder and a reservoir, wherein the wheel cylinder hydraulic pressure can be reduced, held and increased by controlling the pressure-increasing valve and the pressure-reducing valve.
The ABS control is, in general, started in response to the establishment of predetermined ABS control start condition, and is accomplished by performing the pressure-increasing control after the execution of the pressure-reducing control. When the ABS control start condition is again satisfied during the pressure-increasing control in this-time ABS control, the pressure-increasing control is ended and the next ABS control is continuously started. Specifically, supposing that the period from when the ABS control start condition is satisfied to when the next ABS control start condition is satisfied is referred to as one control cycle, the ABS control is, in general, executed continuously plural times over plural-time control cycles.
Recently, there arises a demand of executing the control (hereinafter referred to as “linear pressure-increasing control”) for gently (steplessly) increasing the wheel cylinder pressure during the pressure-increasing control. Therefore, in the brake hydraulic pressure control apparatus, a linear solenoid valve (in particular, normally-open linear solenoid valve) that can (steplessly) control a differential pressure (hereinafter referred to as “actual differential pressure”) between the master cylinder hydraulic pressure and the wheel cylinder hydraulic pressure by linearly controlling the energizing current value has recently been employed as the pressure-increasing valve (e.g., see the following Patent Reference 1). [Patent Reference 1] Japanese Patent Application Laid-Open (kokai) No. 2003-19952.
In general, the aforesaid normally-open linear solenoid valve specifies the relationship between the energizing current value (command current) and the differential pressure (command differential pressure) corresponding to suction force by its specification. When the command differential pressure determined according to the energizing current value is greater than the actual differential pressure, the normally-open solenoid valve is closed to break the communication between the master cylinder and the wheel cylinder. On the other hand, when the command differential pressure is smaller than the actual differential pressure, the normally-open linear solenoid valve is opened to establish the communication between the master cylinder and the wheel cylinder. As a result, the brake hydraulic pressure is flown from the master cylinder into the wheel cylinder, thereby being capable of making an adjustment such that the actual differential pressure agrees with the command differential pressure.
Accordingly, in order to execute the linear pressure-increasing control by using the normally-open solenoid valve as the pressure-increasing valve, at first, the energizing current value to the normally-open linear solenoid valve (i.e., pressure-increasing valve) is immediately set to the current value corresponding to the actual differential pressure (i.e., the energizing current value in order to match the command differential pressure with the actual differential pressure; hereinafter referred to as “current-value-corresponding-to-actual-differential-pressure”) at the point of starting the linear pressure-increasing control, and the energizing current value should be linearly reduced with a constant slope after that, with the pressure-reducing valve maintained in its closed state. Accordingly, the actual differential pressure is gradually decreased after the point of starting the linear pressure-increasing control, with the result that the wheel cylinder hydraulic pressure can be gently increased over the linear pressure-increasing control.
In other words, in order to gently increase the wheel cylinder hydraulic pressure from the point of starting the linear pressure-increasing control, it is necessary to correctly obtain the current-value-corresponding-to-actual-differential-pressure (accordingly, the actual differential pressure at this point) at the point of starting the linear pressure-increasing control (or before this point). The actual differential pressure can easily be detected by using both a sensor detecting the master cylinder hydraulic pressure and a sensor detecting the wheel cylinder hydraulic pressure. However, using these two sensors is generally difficult to be adopted, since there arises a problem of increasing production cost and a problem of being difficult to secure reliability of the sensors.
From the above, it is necessary to obtain the actual differential pressure (or current-value-corresponding-to-actual-differential-pressure) at the point of starting the linear pressure-increasing control without utilizing these sensors. Therefore, the brake hydraulic pressure control apparatus disclosed in the Patent Reference 1 obtains the current-value-corresponding-to-actual-differential-pressure during the linear pressure-increasing control in the first-time control cycle (first-time ABS control), and based upon this value, obtains the current-value-corresponding-to-actual-differential-pressure at the point of starting the linear pressure-increasing control in the second-time and the following control cycles. This technique will be more specifically explained hereinafter with reference to FIGS. 13 and 14.
FIG. 13 is a time chart showing one example of a change in wheel speed Vw, (estimated) vehicle body speed Vso, master cylinder hydraulic pressure Pm, wheel cylinder hydraulic pressure Pw and command current value Id (i.e., energizing current value) to the pressure-increasing valve that is the normally-open linear solenoid valve, in case where a driver of a vehicle having mounted thereto the brake hydraulic pressure control apparatus executing the above-mentioned technique continuously executes the brake operation from a certain point before time t1 and the ABS control start condition is satisfied at time t1 and time t4 (i.e., in case where the period from time t1 to time t4 corresponds to the first-time control cycle and the period after time t4 corresponds to the second-time control cycle).
FIG. 13 shows the case in which the actual differential pressure at the point of starting the linear pressure-increasing control (time t2) becomes relatively small (such braking state is hereinafter referred to as “slow braking”) since the brake operation force (accordingly, master cylinder hydraulic pressure Pm) is kept generally constant immediately after the first-time ABS control starting condition is established.
As shown in FIG. 13, this apparatus starts the pressure-reducing control (pressure-increasing valve: closed; pressure-reducing valve: opened) simultaneous with the start of the first-time ABS control at time t1, and when predetermined holding control start condition is satisfied during this pressure-reducing control, executes the holding control (pressure-increasing valve: closed; pressure-reducing valve: closed) after the pressure-reducing control. Since the pressure-increasing valve (normally-open linear solenoid valve) is kept closed during the execution of the pressure-reducing control and the execution of the holding control, the command current value Id is set to be a certain value (constant value) sufficiently greater than the current-value-corresponding-to-actual-differential-pressure.
Thereafter, since predetermined pressure-increasing control start condition is satisfied upon having reached time t2, this apparatus sets the command current value Id to an initial value at time t2 and linearly decreases the command current value Id with a constant slope in the period from time t2 to time t4, thereby executing the linear pressure-increasing control (pressure-reducing valve: closed). The initial value of the command current value Id is set, for example in the Patent Reference 2, to the current value corresponding to the pressure obtained by adding the increased amount of the master cylinder hydraulic pressure Pm during the period from the point of starting the pressure-reducing control (time t1) to the point of starting the linear pressure-increasing control (time t2) to the actual differential pressure that is increased due to the decrease in the wheel cylinder hydraulic pressure Pw during the pressure-reducing control. [Patent Reference 2] Japanese Patent Application Laid-Open (kokai) No. 9-240451.
In this example, the “slow braking” is performed in which the actual differential pressure at the point of starting the linear pressure-increasing control (time t2) is relatively small, so that the command current value Id becomes greater than the current-value-corresponding-to-actual-differential-pressure (i.e., the command differential pressure is greater than the actual differential pressure) during the period from time t2 to time t3, that means the normally-open linear solenoid valve is maintained in its closed state during the period from time t2 to time t3.
Accordingly, the wheel cylinder hydraulic pressure Pw becomes constant during this period. Upon having reached time t3, the command current value Id agrees with the current-value-corresponding-to-actual-differential-pressure, so that the normally-open linear solenoid valve is opened and the wheel cylinder hydraulic pressure Pw is increasing according to the decrease in the command current value Id during the period from time t3 to time t4. In other words, the command current value Id keeps on agreeing with the current-value-corresponding-to-actual-differential-pressure during the period from time t3 to time t4. Thus, this apparatus can correctly obtain the current-value-corresponding-to-actual-differential-pressure Idc at time t4 that is the point of ending the linear pressure-increasing control.
Subsequently, this apparatus again starts and executes the pressure-reducing control simultaneous with the start of the second-time ABS control at time t4. At this time, this apparatus obtains, by a predetermined technique, a current value (hereinafter referred to as “current-value-corresponding-to-reduced-pressure ΔIrdc) corresponding to the actual differential pressure increasing with the decrease in the wheel cylinder hydraulic pressure Pw during this pressure-reducing control. The current-value-corresponding-to-reduced-pressure ΔIrdc can be obtained, for example, as the product of the pressure-reducing control continuation time by a predetermined coefficient.
After executing the holding control after this pressure-reducing control and upon having reached time t5 that is the point when the linear pressure-increasing control start condition is satisfied, this apparatus sets the command current value Id to a value ID (ID=Idc+ΔIrdc) obtained by adding the aforesaid “current-value-corresponding-to-reduced-pressure ΔIrdc” to the aforesaid “current-value-corresponding-to-actual-differential-pressure Idc at the point of ending the linear pressure-increasing control”. Here, this value ID agrees with the current-value-corresponding-to-actual-differential-pressure at time t5 (i.e., at the point of starting the linear pressure-increasing control in the second-time control cycle).
Therefore, the command current value Id keeps on correctly agreeing with the current-value-corresponding-to-actual-differential-pressure even in the linear pressure-increasing control executed after time t5, like the previous period from time t3 to time t4, with the result that the current-value-corresponding-to-actual-differential-pressure at the point of starting the linear pressure-increasing control in the third-time and the following control cycles can also be successively and correctly obtained like the aforesaid value ID.
As described above, in case where the “slow braking” is applied (more specifically, in case where the command current value Id (i.e., aforesaid initial value) at the point of starting the linear pressure-increasing control becomes greater than the current-value-corresponding-to-actual-differential-pressure), this apparatus correctly obtains the current-value-corresponding-to-actual-differential-pressure at the point of ending the linear pressure-increasing control (i.e., at the point of ending the first-time linear pressure-increasing control) by utilizing the fact that the command current value Id keeps on agreeing with the current-value-corresponding-to-actual-differential-pressure during the period from a certain point (time t3) during the linear pressure-increasing control in the first-time control cycle to the point of ending the linear pressure-increasing control, and based upon this value, this apparatus can correctly obtain the current-value-corresponding-to-actual-differential-pressure at the point of starting the linear pressure-increasing control in the second-time and the following control cycles.
However, in case where the “slow braking” described above is applied, there may arise the period (see period from time t2 to time t3 in FIG. 13) when the normally-open linear solenoid valve is kept closed (i.e., the period when the wheel cylinder hydraulic pressure Pw is held) since the command current value Id is greater than the current-value-corresponding-to-actual-differential-pressure (i.e., the command differential pressure is greater than the actual differential pressure) during the linear pressure-increasing control as described above. In other words, there may arise a problem in which the start of the pressure increase of the wheel cylinder hydraulic pressure Pw is delayed only during this period.
On the other hand, FIG. 14 is a time chart, corresponding to FIG. 13, wherein the actual differential pressure at the point of starting the linear pressure-increasing control (time t2) becomes relatively great as the brake operation force (accordingly, master cylinder hydraulic pressure Pm) increases over a relatively long period even after the first-time ABS control starting condition is established (after time t1) (this braking state is hereinafter referred to as “sudden braking”). Note that the times t1, t2, t3′, t4′, and t5′ in FIG. 14 respectively correspond to the time t1, t2, t3, t4, and t5 in FIG. 13.
The example shown in FIG. 14 indicates the case in which the command current value Id (i.e., the aforesaid initial value) is sufficiently smaller than the current-value-corresponding-to-actual-differential-pressure (i.e., the command differential pressure is sufficiently smaller than the actual differential pressure) at time t2, since the “sudden braking” is applied in which the actual differential pressure at the point of starting the first-time linear pressure-increasing control (time t2) becomes relatively great.
In this case, after time t2, the normally-open linear solenoid valve is kept opened, so that the actual differential pressure is rapidly decreasing toward the command differential pressure according to the command current value Id (i.e., the current-value-corresponding-to-actual-differential-pressure is rapidly decreasing toward the command current value Id). Accordingly, the wheel cylinder hydraulic pressure Pw is rapidly increasing.
Then, upon having reached time t3′ when the current-value-corresponding-to-actual-differential-pressure agrees with the command current value Id, the above-mentioned “rapid increase in the wheel cylinder hydraulic pressure Pw” is ended, whereby the wheel cylinder hydraulic pressure Pw increases with the decrease in the command current value Id during the period from time t3′ to time t4′ that is the point of ending the linear pressure-increasing control, like the period from time t3 to time t4 in FIG. 13. In other words, the command current value Id keeps on agreeing with the current-value-corresponding-to-actual-differential-pressure during the period from time t3′ to time t4′. Accordingly, this apparatus can correctly obtain the current-value-corresponding-to-actual-differential-pressure Idc at the point of ending the first-time linear pressure-increasing control (time t4′) (accordingly, the current-value-corresponding-to-actual-differential-pressure ID at the point of starting the second-time linear pressure-increasing control (time t5′)).
As described above, even in case where the “sudden braking” is applied (more specifically, in case where the command current value Id (i.e., aforesaid initial value) at the point of starting the linear pressure-increasing control becomes smaller than the current-value-corresponding-to-actual-differential-pressure), this apparatus correctly obtains the current-value-corresponding-to-actual-differential-pressure at the point of ending the linear pressure-increasing control (i.e., at the point of ending the first-time linear pressure-increasing control) by utilizing the fact that the command current value Id keeps on agreeing with the current-value-corresponding-to-actual-differential-pressure during the period from a certain point (time t3′) during the linear pressure-increasing control in the first-time control cycle to the point of ending the linear pressure-increasing control, and based upon this value, this apparatus can correctly obtain the current-value-corresponding-to-actual-differential-pressure at the point of starting the linear pressure-increasing control in the second-time and the following control cycles.
However, in case where the “sudden braking” described above is applied, there may arise a problem in which wheel cylinder hydraulic pressure Pw rapidly increases over a relatively long period caused by the command current value Id sufficiently smaller than the current-value-corresponding-to-actual-differential-pressure (i.e., the command differential pressure is sufficiently smaller than the actual differential pressure) during the linear pressure-increasing control as described above.
As understood from the above, the brake hydraulic pressure control apparatus disclosed in the Patent Reference 1 entails a problem in which the start of pressure increase of the wheel cylinder hydraulic pressure may be delayed during the linear pressure-increasing control in case where the “slow braking” is applied, and the wheel cylinder hydraulic pressure rapidly increases over a relatively long period during the linear pressure-increasing control in case where the “sudden braking” is applied.